The objective of the study was to determine whether coitus during menstruation in perimenopausal women is associated with increased menstrual flow defined as either heavier flow, more days of flow, or both. Initial interviews for the Stanford Menopause Study were conducted locally with 160 perimenopausal women. Six weeks later, additional data were gathered from 121 women. At a third interview, 6-10 weeks later at Stanford University, 56 of 57 subjects (mean age 48.8 +/- 3.9 SD) provided data on menstrual flow and coital behavior. At both the first and third interview, more than half of the women reported a pattern of increased menstrual flow. At the third interview, 83% of those reporting increased menstrual flow also reported a behavioral pattern of coitus during menses. Only 10% with diminished flow acknowledged such a pattern. Women who abstained from sex during menses were not less regular in coital activity at other times nor did the data reveal them to be less orgasmic than women who did not abstain. Our data reveal an association between coitus during menstruation and longer and/or heavier menstrual flow. The etiology of such increased flow is usually believed to be hormonal, but coitus during menstruation may be another important variable.